1. Field of the Invention
The present invention pertains to the method and devices designed and constructed to test transformers. In particular, the present invention describes a method and apparatus for testing resistances of transformer windings and configurations of connected transformers windings. The invention has a broad applicability for testing of transformers of different sizes and power ratings.
2. Description of the Prior Art
Measurements of transformer winding resistances are common transformer diagnostic techniques. It is used to detect variety of transformer failures related to damage of winding conductors, terminals, or insulation.
Measurement of transformer winding resistance is inherently non-trivial because of the high inductance required for the basic function of the transformer. Some power transformers may exhibit inductances in excess of 2000H. Voltage drops over transformer windings are usually dominated by the inductive L(i)di/dt contribution that can exceed the resistive Ri contribution by several orders of magnitude.
The presence of the ferromagnetic transformer core, which is a standard in the transformer art, further complicates the problem of winding resistance measurements by introduction of nonlinear dependence of the inductance on the transformer current and the magnetic properties of the ferromagnetic core material. Characterization of magnetic properties of a ferromagnetic substance can be achieved using magnetic permeability μ. The simplest description of ferromagnetic substances utilize the magnetic permeability μ as a dimensionless scalar multiplier (in the Gaussian cgs system of units)describing the enhancement of the magnetic flux density in the ferromagnetic material over the flux density in a vacuum produced by an equivalent source of the magnetic field. Ferromagnetic materials, like “Supermalloy”, can exhibit maximal μ=1,000,000, but reach saturation in high magnetic fields which asymptotically brings the magnetic permeability back to the vacuum value of 1.
The phenomenon of ferromagnetic saturation plays an important role in the prior art high current transformer resistance meters. In order to maximize resistive voltage drop Ri over a transformer winding and simultaneously minimize the inductive L(i)di/dt contribution to the voltage drop, the measuring devices of the prior art are designed to operate in the high current regime. For example, the High Current Resistance Meter Type 2292 of Tettex Instruments is designed to deliver maximum DC test current of 50A.
The high current testers of the prior art are large and heavy devices utilizing massive, heavily insulated conductors and connectors. Operation in the vicinity of the saturation of the transformer core exposes the operators and the equipment to the hazards of accidental discharge of the significant magnetization energy stored in the transformer core. In addition, the measurement process takes a relatively long time to bring the core near saturation to cause the L(i)di/dt voltage to be small enough to provide valid measurements by reducing L and di/dt to negligible levels. A similarly long time must be spent to safely discharge and dissipate the stored energy.
Alternative methods of transformer parameter testing are known:
US Patent Application 2004/0130329 corresponding to the European Patent EP1398644, by Suss, describes a method of transformer testing according to the IEC 60044-6 Standard. The described method for testing instrument transformers utilizes transformer test signals of more than one frequency (that can be lower that the operating frequency of the transformer) to obtain a set of transformer frequency-dependent responses, which are used to derive a simulation model of the transformer. The resulting simulation model is used for calculation of the transformer parameters required by the IEC 60044-6 Standard.
The U.S. Pat. No. 6,608,493 by Thomas et al. describes an automatic portable transformer tester. The essence of this invention is the use of a matching transformer between a computerized signal source and the tested device. The matching transformer expends the testing parameter space covered by a compact “portable” signal source.
Russian Patent RU2192020 by Nefed, protects usage of external “frequency independent coaxial shunt” and a current comparator in current transformers testing. This is an illustration of the testing methods based on addition of known external components to the measured transformer in order to execute the measurements or facilitate the measured data analysis.
U.S. Pat. No. 5,276,402 by Schuch, describes relatively conventional method of “ANSI-required” three phase transformer testing, based on simultaneous monitoring of six parameters associated with three phases. The emphasis is on convenience, ease of usage, and operator error avoidance.
Japanese Patent JP5157795 by Komatsu protects measuring method which involves switchable “signal source 2” that can switch power of the test signal between primary or secondary windings of the tested transformer. The method eliminates the need for discharging and disconnecting the measured transformer when the measurements on primary and secondary sides of the transformer are needed.
Japanese Patent JP60219564 by Azuma et al. is an illustration of the transformer testing methods based on applications of external bridge circuits. Bridges are relatively common in transformer diagnostics prior art in spite of the inherent complexity of the method and complicated analysis and processing of the measurement results.
The above identified methods of transformer diagnostics are only examples of well-developed prior art of specialized testing of specific subgroups of transformers. This prior art differs from the present invention by relative complexity and focused applicability of the protected methods and apparatus.